Swimmers and dog owners are being told to avoid Lake Waihola and parts of the Mataura River as the hot weather brings elevated levels of toxic algae.

Monitoring by Environment Southland has found higher levels of the toxic algae benthic cyanobacteria in the river near the Mataura Island Bridge.

Environment Southland director of science and information Graham Sevicke-Jones said because of the warm weather it's likely the algae will be present in different areas along the river and possibly other rivers in Southland.

“People should take extra care when choosing a swimming spot or letting dogs off leads along rivers,” he said.

These algae can produce toxins that are harmful to people and animals if swallowed, or through contact with the skin, he said.

People and animals (dogs in particular) should avoid contact with the water and any mats that have formed at the waters’ edge.

The Otago Regional Council has also put out a warning today saying people and their pets should stay out of the water at Lake Waihola.

It posted a warning online saying a colourful algae in the lake was potentially toxic blue-green algae (cyanobacteria). It advised people and their pets avoid contact with the water and follow the instructions on any warning signs around the lake, until further notice.

Benthic cyanobacteria can be recognised at rivers and streams as a green/brown slime on rocks, or dark brown/black mats at the water’s edge. Several factors can contribute to the growth of toxic algae, such as high levels of nutrients or sediment run-off into waterways, a sudden increase in temperature, and low flows.